What is the third parameter of a Go struct field?

Question

type Config struct {
    CommitIndex uint64 `json:\"commitIndex\"`
    // TODO decide what we need to store in peer struct
    Peers []*Peer `json:\"peers\"`
}


        

Answer 1

What you are referring to is called a tag, and the Go specification states:

A field declaration may be followed by an optional string literal tag, which becomes an attribute for all the fields in the corresponding field declaration. The tags are made visible through a reflection interface and take part in type identity for structs but are otherwise ignored.

// A struct corresponding to the TimeStamp protocol buffer.
// The tag strings define the protocol buffer field numbers.
struct {
    microsec  uint64 "field 1"
    serverIP6 uint64 "field 2"
    process   string "field 3"
}

This does nothing at compile time, but is used by different packages when doing runtime reflection on the struct. As Amit already pointed out, the encoding/json package is using it to specify marshalling/unmarshalling behaviour. The same goes with encoding/xml, gopkg.in/mgo.v2/bson, etc.

The tag string is by convention a space separated string. As stated in the reflect package:

By convention, tag strings are a concatenation of optionally space-separated key:"value" pairs. Each key is a non-empty string consisting of non-control characters other than space (U+0020 ' '), quote (U+0022 '"'), and colon (U+003A ':'). Each value is quoted using U+0022 '"' characters and Go string literal syntax.

Answer 2

It's called a struct tag, they can be parsed using the reflect package at runtime.

From https://golang.org/ref/spec#Struct_types:

A field declaration may be followed by an optional string literal tag, which becomes an attribute for all the fields in the corresponding field declaration.

The tags are made visible through a reflection interface and take part in type identity for structs but are otherwise ignored.

Some packages that use reflection like json and xml use tags to handle special cases better.